Method of nitriding metallic surfaces



Jan, 29, 1957 R. 1 CHENAULT ETAL 2,779,697

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/NVENTORS METHUD QF NITRIDING METALLIC SURFACES Roy L.. Chenault, Seneca, and Gerald E. Mohnkern,

Qi! City, Pa., assignors to United States Steel Corporation, a corporation of New Jersey Application September 26, 1955, Serial No. 536,724

4 Claims. (Cl. 14S-16.6)

This invention relates to improved methods of nitriding metallic surfaces.

The present application is a continuation-in-part of our earlier co-pendng application Serial No. 276,366, tiled March 13, 1952, now abandoned. Our earlier ap` plication was in turn co-pending with the application which resulted in our Patent No. 2,596,981, issued May 20, 1952. This patent shows and claims improved nitriding methods in which a capsule of liquid ammonia is sealed within a hollow workpiece, for example a length of steel tubing from which a pump barrel is to be formed. The workpiece is heated to a temperature approximately between 800 and 1200 F., at which the capsule releases its ammonia in gaseous form to the interior of the workpiece, where it is confined under relatively high pressure. Under such conditions ammonia dissociates to form nascent nitrogen and hydrogen, but the reaction is reversible. Consequently, part of the nascent nitrogen combines with the surface of the workpieceto nitride and case-harden this surface, while the remainder resumes its molecular state, in which it is ineffective for nitriding, but recombines with hydrogen to form more ammonia.

We have found that the method as actually disclosed in our patent is particularly well suited for nitriding interior surfaces of tubular articles, such as pump barrels. Nevertheless for nitriding certain other types of articles it may be preferable to dispense with the capsule and introduce ammonia to the nitriding chamber as a liquid or vapor from a source externally of the chamber. The workpiece then is heated as before and similar reactions take place. Such methods are well suited for nitriding surfaces of smaller articles which can be placed within a sealed nitriding chamber. Accordingly an object of the present invention is to provide improved nitriding methods in which ammonia is charged to a sealed chamber from a source externally thereof and is confined in said chamber in direct contact with the surfaces to be nitrided and heated under conditions` such that its dissociation reaction is reversible.

A further object is to provide improved methods which utilize only the original ammonia charge as a source of nitrogen and yet dispense with the need for a capsule for charging this ammonia.

Conventional nitriding methods in which ammonia is circulated over the work often produce a white layer on the nitrided surface. This layer is a chemical compound richer in nitrogen than the interior of the case, and is undesirable for most purposes. A further object of the present invention is to control or prevent formation of a white layer by positively regulating the mass of ammonia per unit area to include values lower than disclosed in our prior patent.

ln accomplishing these and other objects of the invention, we have provided improved details of structure, preferred forms of which are shown in the accompanying drawings, in which:

Figure l is a schematic vertical sectional view of one nited States PatentF ice form of apparatus for carrying out the nitriding method of the present invention;

Figure 2 is a schematic horizontal sectional view of a modified form of such apparatus;

Figure 3 is a schematic vertical sectional view of another modification;

Figure 4 is a schematic side elevational view of an apparatus for charging liquid ammonia to a metering vessel, representing still another modification;

Figure 5 is a schematic side elevational view of the apparatus of Figure 4 while charging ammonia from the metering vessel to the nitriding chamber;

Figure 6 is a graph which shows experimentally determined relations between the mass of ammonia per unit area and case characteristics with a typical aluminum and chromium bearing nitriding steel;

Figure 7 is a graph similar to `Figure 6 but with an aluminum, chromium and nickel bearing nitriding steel; and

Figure 8 is another graph similar to Figure 6 with A. I. S. I. 4140 steel (chromium and molybdenum bearing).

`Figure l shows schematically a nitriding apparatus which comprises a furnace 10, a sealed nitriding charnber 12 within said furnace, an ammonia supply tank 13, a water jacket 14 surrounding said tank, and a pipe 15 connected between chamber 12 and tank 13. This tank is equipped with the usual valve 16. The water jacket 14 has a water inlet 1'7 and outlet 18. Pipe 15 contains a detachable union 19, a valve 20 and a gauge 21 which can measure either pressure or flow. Chamber 12 can contain workpieces 22 to be nitrided or else this chamber itself can be a workpiece whose interior surface is to be nitrided. The furnace 10 and chamber 12 can be heated by any standard or desired means, not shown.

Tank 13 contains both liquid ammonia and ammonia vapor under pressure. The water within the jacket 14 is maintained at a controlled temperature to regulate the vapor pressure within the tank. On opening of valves 16 and 20, ammonia vapor iiows from tank 13 into chamber 12. The pressure or flow indicated on gauge 21 furnishes a measure of the mass of ammonia thus charged to the chamber, the volume and temperature of the chamber being known. The chamber temperature must be above the tank temperature to prevent an uncontrolled accumulation of liquid ammonia in the chamber during charging. By way of example, if the water temperature surrounding the tank 14 is 190 F., the vapor pressure of liquid ammonia within the tank is 709 p. s. i. absolute, or at room temperature this vapor pressure is approxi mately p. s. i. absolute. In either instance at least as great a pressure can be transmitted to the chamber 12 as long as the temperature of the latter is above that of the liquid ammonia in the tank. Ammonia vapor in the nitriding chamber is in a superheated condition with its pressure determined by the vapor pressure of liquid ammonia in the tank. Theway in which we determine the mass of ammonia to be charged is explained hereinafter.

After the desired charge of ammonia. has been introduced to the chamber 12, valves `20 and 16 can be closed and chamber 12 heated to the nitriding temperature, approximately 800 to 1200 `F. These limits are fairly critical; surfaces nitrided at 1250 F. or higher show no appreciable hardening and hence such temperatures are maintained at the nitriding temperature a sutiiciently long` period for the desired degree of nitriding to take place, which interval of course varies with conditions, but about hours can be considered average. In common with the method disclosed in our patent, only the original ammonia charge is used for nitriding, and the reversiblity of the dissociation reaction assures a continuous supply of nascent nitrogen.

Quantitatively we find we can nitride satisfactorily with ammonia charges ranging from one gram per square foot of reactive. surface upwardly, they exact charge depending on the `case characteristics desired and on the material being nitrided. Referring to Figure 6, a typical aluminrun-chromium nitriding steel acquires no. white layer with an ammonia mass up to more than 9 grams per square foot, of reactive surface. This figure also shows that cases of practical depth and hardness for commercial usey are obtained throughout the full range of ammonia mass to area ratios plotted therein. For the higher ratios some white layer results, but there is a corresponding increase in case depth and hardness. These higher ratios can be utilized effectively when a relatively slight white layer is not objectionable. Figures 7 and 8 show similar data for two other types of steel. A. I. S. I. 4140 steel shown in Figure 8 is not ordinarily considered a nitriding steel, yet by our method we were able to produce a satisfactory nitrided case thereon.

Figure 2 shows schematically a modified apparatus in which the furnace 10 houses several nitriding chambers 12a which are charged simultaneously with ammonia from a single tank 13. If desired, this tank can be placed in a water jacket as in Figure l. A manifold 23 is connected to the tank with a valve 24 and pressure gauge 25 interposed therebetween. Branch pipes 26, which contain detachable unions 27, connect said manifold with the various nitriding chambers 12a. The procedure for charging ammonia as explained in the description of Figure 1 can be followed for charging this modified apparatus. However, we have shown a vacuum pump 2S connected via valves 29 and a pressure gauge 30 to the end of the manifold 23 opposite the tank 13. This pumpV can be used to exhaust air from the nitriding chambers before introducing ammonia thereto. We have found we obtain a cleaner nitrided case if the air is exhausted from the chambers. It is also desirable to exhaust the air when ther chamber volume is large in relation to the reactive surface area. It is apparent also that the embodiment shown in Figure 1 could be similarly equipped with a vacuum pump. Equivalent benefits also have been obtained by purging the chamber with nitrogen.

Figure 3 shows another modified apparatus which charges gaseous ammonia in controlled quantities to a nitriding chamber 31. The apparatus includes an ammonia tank 32, an intermediate reservoir 33 and piping 34 connecting the tank, reservoir and nitriding chamber as shown. rThe piping contains three valves 35, 36 and 37, which are adapted to close off the chamber, reservoir and tank respectively, a pressure gauge 3S, which is adapted to indicate the pressure in the reservoir, and detachable unions 39. Both the tank 32 and reservoir 33 are situated, within Water jackets i0 and 41 respectively. The nitriding chamber 31 is situated within a furnace not shown. It is illustrated as including a grid support 42, on which workpieces 43 to be nitrided are placed, a cooling coil 44, and an insulating bottom 45.

Before ammonia is charged to the nitriding chamber 31, it is charged in vapor form from tank 32 to the intermediate reservoir 33. 'Ihis rst charging is accomplished by opening the Valves 36 and 37 and closing valve 35. Both the tank and reservoir are heated by use of their respective water jackets 40 and 41, with the latter heated to a little higher temperature than the former. Thus ammonia vapor flows from the tank to the reservoir, and the two reach ythe same pressure, which is indicated on the gauge 38. The vapor pressure and temperature of 4 ammonia in reservoir 33 and volume of this reservoir furnish a measure of the quantity of ammonia therein.

Next valve 37 is closed and valve 35 opened. The nitriding chamber 31 either is at room temperature or below, being cooled by circulation of refrigerant in its cooling coil 44. The majority of the ammonia Vapor in reservoir 33 thus flows into the nitriding chamber 31, where some of it may iquify. The valve 35 is then closed and the chamber heated as in the other embodiments to effect nitriding of the surfaces of the workpieces.

Figures 4 and 5 show schematically a further modification in which ammonia is charged from a tank 46 into a metering Vessel 47 (Figure 4) and thence into a nitriding chamber 48 (Figure 5). The apparatus for charging the vessel 47. includes pipe segments 49 and 49a, which contain valves 50 and 51 respectively and are joined by a detachable union 52. Pipe segment 49a extends to the l-ower part of tank 46. Preferably a vacuum pump 53 is connectedto pipe 49 via a valve 54 and a detachable union 55. Preferably the vessel 47 is transparent and carries graduations 56.

With the vessel charging apparatus assembled as shown in Figure, 4 first the valves 50 and 54 are opened with valve 51 closed. The pump 53 is operated to exhaust the air from the vessel 47. Next the valve 54 is closed and valve 51 opened. Vapor pressure. on the liquid ammonia in tank 46 forces Ithis liquid through the pipe 49, 49a into the vessel 47. When the. liquid ammonia reaches the desired level in said vessel, valves 50 and 51 are closed, and the pipe segments 49 and 49a are detached from each other.

Next the vessel 47 and its pipe segment 49 are transferred to the nitriding chamber 48. The latter is equipped with a pipe segment 57 which has a union half to which the union half 52 can be attached. The segment 57 also has a valve 58 and preferably is equipped with a vacuum pump 59, which is used to exhaust air from the nitriding chamber. The desired quantity of ammonia is transferred from vessel 47 into chamber 48 by opening valves 50 and 58. Next the valve 58 and a valve 60 leading to the vacuum pump are closed. The actual nitriding then is effected by heating the chamber as in the other embodiments already described.

From the foregoing description it is seen that the present invention affords nitriding methods similar in principle to that disclosed in our earlier application, but in each instance the ammonia is charged from a source externally of the nitriding chamber. Thus the need for a special capsule is eliminated and also the need for specially welding plates to the workpiece to furnish the necessary seal. We have further improved on the method disclosed in our patent byl positively relating the mass of ammonia charged to the chamber with the area of reactive surface to control the formation of white layer.

While We have shown and described preferred embodiments of the invention, it is apparent that modifications may arise. Therefore, we do not wish to be limited to the disclosure set forth but only by the scope of the appended claims.

We claim:

1. A method `of nitriding surfaces of metallic articles and controlling the formation of white layer thereon comprising placing the surfaces to be nitrided within a chamber, connecting said chamber to a source of ammonia located externally thereof, heating the chamber to a ternpera'ture above that of the ammonia source, charging a regulated mass `of ammonia per unit area vof surface reactive to nitrogen from said source into said chamber and into direct contact with said. surfaces, said mass being equivalent to at least one gram of ammonia per square foot of reactive surface but below the mass which produces appreciable white layer, sealing said chamber with said surfaces and said ammonia in contact and confined therein, and heating said chamber to a temperature approximately between 800 and 1200 F. for a prolonged period.

2. A method as defined in claim 1 in which the original charge of ammonia constitutes the entire charge.

3. A method `of nitriding surfaces of aluminum and chromium bearing nitriding steel and avoiding the formation of White layer thereon comprising placing the surfaces to be nitrided within a chamber, connecting said chamber lo a source of ammonia located externally thereof, heating the chamber to a temperature above that `of `the arnmonia source, charging a regulated mass of ammonia per unit area of `surface reactive to nitrogen from said source into said chamber and into direct contact with -said surfaces, said mass being in the ratio of about one -to nine grams `of ammonia per square foot of reactive surface and constituting the entire ammonia charge, sealing said chamber with said ammonia and said surfaces in Contact and conned therein, and heating `said chamber to a temperature approximately between 800 and 1200 F. for approximately l5 hours.

4. A method of nitriding surfaces of metallic articles and `controlling formation of white layer thereon comprising placing said `surfaces Within a chamber, metering a mass of liquid ammonia by transferring it under pressure and at an increasing temperature from a source external to ysaid chamber, vsaid metered mass `being in the ratio of about one to nine grams of ammonia per square foot of reactive surface, admitting Vsaid metered mass directly into said chamber as the sole charge lof ammonia thereto, sealing said chamber with said `ammonia and said surfaces in direct contact 'an-d conned therein, `and heating the sealed chamber to a temperature approximately between 800 and i200 F. for a prolonged period.

References Cited in the file of this patent UNITED STATES PATENTS 2,452,915 Feild Nov. 2, 1948 2,596,981 Chenault et al. May 20, 1952 FOREIGN PATENTS 492,663 Great Britain Sept. 23, 1938 

1. A METHOD OF NITRIDING SURFACES OF METALLIC ARTICLES AND CONTROLLING THE FORMATION OF WHITE LAYER THEREON COMPRISING PLACING THE SURFACES TO BE NITRIDED WITHIN A CHAMBER, CONNECTING SAID CHAMBER TO A SOURCE OF AMMONIA LOCATED EXTERNALLY THEREOF, HEATING THE CHAMBER TO A TEMPERATURE ABOVE THAT OF THE AMMONIA SOURCE, CHARGING A REGULATED MASS OF AMMONIA PER UNIT AREA OF SURFACE REACTIVE TO NITROGEN FROM SAID SOURCE INTO SAID CHAMBER AND INTO DIRECT CONTACT WITH SAID SURFACES, SAID MASS BEING EQUIVALENT TO AT LEAST ONE GRAM OF AMMONIA PER SQUARE FOOT OF REACTIVE SURFACE BUT BELOW THE MASS WHICH PRODUCES APPRECIABLE WHITE LAYER, SEALING SAID CHAMBER WITH SAID SURFACES AND SAID AMMONIA IN CONTACT AND CONFINED THEREIN, AND HEATING SAID CHAMBER TO A TEMPERATURE APPROXIMATELY BETWEEN 800 AND 1200F. FOR A PROLONGED PERIOD. 